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Discussion Starter #1
Hoping someone can help. I know zx12s run hot and have issues with heating up but what mine is doing can’t be normal.

on a run at any speed the temp is fine, either half way or just below it, if I’m stopped in traffic for 30 seconds, it’s up into the red and boiling the coolant out the over flow.

iv done the follow checks

water pump working 100%
fans working
Rad, flushed and seems fine
Thermostat (haven’t checked because my hands aren’t that small, but if it’s running ok normally I’m thinking it’s working ok)
Bleed the system as while so there isn’t any air pockets
Bike isn’t pressurising, even at really high temp I can still squeeze the water pipes.

what im trying next, running water wetter and wiring one fan upto a switch I can control myself.

Anyone any other advice or ideas, thanks
 

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You've got to replace the thermostat with an OEM unit. Could it be stuck slightly open? Weird the hoses arent pressurized... how did you measure water pump activity and flow?
 

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Discussion Starter #3
Thanks for replying. The thermostat has been pointed out by a few people, although when it was removed it seems fine and working when I did the boiling water test. Although I have ordered an oem part.
Removing the old one wasn’t so nice though, one of the bolts snapped so I know have the engine removed to fix this.
16610
34A1F9BA-55DD-4D3B-B92D-9C69D7D943BC.jpeg
 

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Discussion Starter #4
As for the water pump, as the bike was running so well normally I wasn’t too concerned about checking the flow, more concerned on the condition of the water pump, although I did remover the top hose from the pump and start the bike so I could watch the pump moving freely, even under throttle it was still spinning freely, so I was happy enough with it, without going much more in-depth.

my plan going forward is to fit a 40% more aluminium rad, new thermostat,use water wetter and wire one fan to a switch for manual control.

if this doesn’t cure the problem I will have to look more in-depth at the water pump
 

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Switch to 100% engine ice. You will need 2 containers to fill from empty.

If the hoses are not obtaining pressure, it is 100% the thermostat. Temp is rising because it is stuck open. Yes, it's tight and you need to take your time, but it's much cheaper than replacing the whole engine from overheating. Use an OEM Kawasaki thermostat.

Also, look into a switch to turn the fan on when you hit heavy traffic.
 

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GET A NEW RAD CAP
you already answered your own question, its not building pressure, it will boil at a lower temp.
 

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Agreed. New radiator cap is the only thing not listed in the first post!
 

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Did you figure it out? Cap does seem like a possible issue.
 

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Maybe a little late to the party but I'm having the same issue as you with my 12. Replaced the thermostat with one from my other engine which I know was good as well as replaced the radiator cap with a new OEM one. Cooling is better but still the hose doesn't feel pressurised. So if you found out what worked for you I'm eager to hear.
 

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Discussion Starter #12
Hey! I’m not sure what actually cured it for me, I was so deep into replacing the thermostat with the broken bolt that while the engine was out I replaced everything.
thermostat
Replaced the rad with a 40% extra aluminium rad which came with its own pressure cap
I also wired the fans upto a switch just for when I’m sitting in traffic for a long time.
I also replaced oil, oil filter, air filters, plugs, Chain and sprockets, although not relevant to the heating issue.
The bike seems to run around half way most times unless I’m stationary for quite some time it will then raise up to 3/4, at that point I switch the fans on myself.
16865
662601BA-183A-48EA-AB71-05871C4C0953.jpeg Just to show the difference between the aluminium and the standard rad
 

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Hey! I’m not sure what actually cured it for me, I was so deep into replacing the thermostat with the broken bolt that while the engine was out I replaced everything.
thermostat
Replaced the rad with a 40% extra aluminium rad which came with its own pressure cap
I also wired the fans upto a switch just for when I’m sitting in traffic for a long time.
I also replaced oil, oil filter, air filters, plugs, Chain and sprockets, although not relevant to the heating issue.
The bike seems to run around half way most times unless I’m stationary for quite some time it will then raise up to 3/4, at that point I switch the fans on myself.
View attachment 16865 View attachment 16865 Just to show the difference between the aluminium and the standard rad
Where did you get that radiator?

Sent from my Pixel 3 XL using Tapatalk
 

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Just had the same issue last week. Low mileage 2001 ZX12R. Got back from a 10 minute ride and when I shut the bike off, I could hear the coolant boiling in the reservoir. Let the bike cool off and next day, started bike and let idle until it warmed up. Watched the coolant rise in reservoir and then start boiling. Bought a 1.6 aftermarket cap (stock is 1.3) to replace original cap and problem was gone. Seal looked good in the old cap but I think the vacuum port that lets fluid return to the radiator when the motor cools was stuck open. Easy and cheap fix in the end.
 
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